Human tumor necrosis factors-alpha (TNF-alpha) and beta (TNF-beta or lymphotoxin) are related members of a broad class of polypeptide mediators, which includes the interferons, interleukins and growth factors, collectively called cytokines (Beutler, B. and Cerami, A., Annu. Rev. Immunol., 7:625-655 (1989)).
Tumor necrosis factor (TNF-alpha and TNF-beta) was originally discovered as a result of its anti-tumor activity, however, now it is recognized as a pleiotropic cytokine playing important roles in immune regulation and inflammation. To date, there are eight known members of the TNF-related cytokine family, TNF-alpha, TNF-beta (lymphotoxin (LT)-alpha), LT-beta, and ligands for the Fas, CD30, CD27, CD40 and 4-1BB receptors. These proteins have conserved C-terminal sequences and variable N-terminal sequences which are often used as membrane anchors, with the exception of TNF-beta. Both TNF-alpha and TNF-beta function as homotrimers when they bind to TNF receptors.
TNF is produced by a number of cell types, including monocytes, fibroblasts, T-cells, natural killer (NK) cells and predominately by activated machrophages. TNF-alpha has been reported to have a role in the rapid necrosis of tumors, immunostimulation, autoimmune disease, graft rejection, resistance to parasites, producing an anti-viral response, septic shock, growth regulation, vascular endothelium effects and metabolic effects. TNF-alpha also triggers endothelial cells to secrete various factors, including PAI-1, IL-1, GM-CSF and IL-6 to promote cell proliferation. In addition, TNF-alpha up-regulates various cell adhesion molecules such as E-Selectin, ICAM-1 and VCAM-1. TNF-alpha and Fas ligand have also been shown to induce programmed cell death.
The first step in the induction of the various cellular responses mediated by TNF or LT is their binding to specific cell surface receptors. Two distinct TNF receptors of approximately 55-KDa (TNF-R1) and 75-KDa (TNF-R2) have been identified (Hohman, H. P. et al., J. Biol. Chem., 264:14927-14934 (1989)), and human and mouse cDNAs corresponding to both receptor types have been isolated and characterized (Loetscher, H. et al., Cell, 61:351 (1990)). Both TNF-Rs share the typical structure of cell surface receptors including extracellular, transmembrane and intracellular regions.
The endothelium, which under physiological conditions is mostly a quiescent tissue (Denekamp, J. Cancer Metas. Rev. 9:267-282 (1990)), plays an essential role in the maintenance of vascular homeostasis and permeability. Endothelial cells are actively involved in inflammation, cell adhesion, coagulation, thrombosis, fibrinolysis, and angiogenesis. During angiogenesis, endothelial cells proliferate, invade into stroma, migrate toward the source of an angiogenesis stimulus, such as cancer cells, interact with perivascular cells and stromal cells, and eventually, form capillary vessels linking the tumor tissue to the circulatory system (Folkman, J. Nature Med. 1:27-31 (1995)). Although the complex mechanism that regulates angiogenesis is yet to be fully understood, it is becoming clear that the initiation or termination of the process is a result of a balance between positive and negative factors.
A number of angiogenic factors, often markedly upregulated in tumor tissues, have been described. These include several members of the fibroblast growth factor (FGF) family, such as FGF-1, FGF-2, and those of the vascular endothelial cell growth factor (VEGF) family and the receptors for all of these molecules (Gimenez-Gallego, G, et al., Science 230:1385-1388 (1985); Schweigerer, L., et al., Nature 325:257-259 (1987); Leung, D. W., et al., Science 246:1306-1309 (1989); Burrus, L. W. and Olwin, B. B. J. Biol. Chem. 264:18647-18653 (1989); Wennstrom, S., et al., Growth Factors 4:197-208 (1991); Terman, B. I., et al., Biochem. Biophys. Res. Comm. 187:1579-1586 (1992); de Vries, C., et al., Science 255:989-991 (1992)). Likewise, several inhibitors of angiogenesis have also been reported, including thrombospondin, angiostatin, endostatin, and platelet factor-4 (Good, D. J., et al., Proc. Natl. Acad. Sci. USA 87:6623-6628 (1990); O'Reilly, M. S., et al., Cell 79:315-328 (1994); O'Reilly, M. S., et al., Cell 88:277-285 (1997); Maione, T. E., et al., Science 247:77-79 (1990)). It is apparent that normal angiogenesis is promptly activated when needed, and swiftly terminated when no longer required. However, pathological angiogenesis, once initiated, is often prolonged and often difficult to stop. This may indicate that a negative regulatory mechanism normally functioning is missing or suppressed in a pathological angiogenic process. It is conceivable that endothelial cells may produce autocrine factors to suppress an angiogenesis process or maintain the quiescence of a mature vasculature.
The polypeptide of the present invention has been identified as a novel member of the TNF family based on structural, amino acid sequence homology, and functional similarities, for example, TNF-gamma is a pro-inflammatory protein. Further, the TNF-gamma polypeptide of the present invention is a negative regulator of angiogenesis and of endothelial cell growth. There is a need for polypeptides that function in this manner, since disturbances of such regulation may be involved in disorders relating to angiogenesis, hemostasis, tumor metastasis, cellular migration, and cancers of many systems. Therefore, there is a need for identification and characterization of such human polypeptides which can play a role in detecting, preventing, ameliorating or correcting such disorders.